Class-switch rearrangements with retention of the Cμ-Cδ region on the nontranslocated, functional alleles of nine IgM expressing FL and one IgH-negative FL (FL14). The positions of Sμ and the Cγ and C plasmid probe signals are indicated. The differentiation between Cγ and C probe signals has been made by sequentially hybridizing probe mixes with and without the C probe. The position of the U2-2 cosmid in FL14 was determined by sequentially hybridizing the IgH probe mix with and without U2-2. Its orientation could be determined by the presence of a Sμ signal derived from the C probe (indicated with an arrow).